1. Field of the invention
The present invention relates to a method and device for processing a signal, for use in communication systems or satellite navigation systems, for instance. More particularly, the present invention relates to a method and device for processing a signal wherein any errors caused by multipath are eliminated or at least reduced. In one particular embodiment, the present invention relates to a navigation system.
2. Description of the Prior Art
In principle, communication from a transmitter to a receiver involves the transmission, by the transmitter, of a carrier wave signal which is coded by an information signal, such as by AM or FM, and the reception, by the receiver, of the coded signal. In the receiver, the received signal is decoded for obtaining the information therein. The propagation time of the signal, i.e., the time which passes between the moment of transmission and the moment of reception, depends upon the distance between the transmitter and the receiver, and upon the propagation speed of the carrier wave signal, as is well known.
In practice, the carrier wave signal reaches the receiver not only directly, i.e., via the line of sight, but also via one or more reflections from objects such as buildings, mountains, clouds, atmospheric layers, etc. Since the propagation path via reflection is always longer than the line of sight, the propagation time of the signal via reflection is always longer than the propagation time of the signal via the line of sight, i.e., a reflected signal always has a certain delay with respect to the direct signal. The receiver can not distinguish between the direct signal and a reflected signal, and the combination of the direct signal as received by the receiver and the reflected signals as received by the receiver is processed as if it were an undistorted signal; however, said combination obviously contains an error with respect to the signal as transmitted. This problem is commonly known as "multipath". Multipath can also be caused by diffraction.
Communication of the above-described kind is commonly used for many applications, and in most applications it is a general desire to reduce the errors caused by multipath. However, one application where reducing such errors is extremely important is navigation, such as satellite navigation, and the present invention will be described with reference to a navigation system, although it is explicitly noted that the invention is not restricted to such use.
In the context of the present application, navigation is based on the principle of determining the distance between an object and a reference position by means of a communication signal. A reference station, such as a satellite, emits a signal, which is received by a receiver associated with said object. By measuring the propagation time of the signal, the distance of the object to the reference station can be calculated. Similarly, the distances of the object to a second and possibly further reference stations are calculated. In the case of a stationary reference station, the position of the reference station is constant and may be incorporated in the receiver as a constant value. In the case of a non-stationary reference station such as a satellite, the signal may contain information regarding the position of the reference station. Since, therefore, the positions of the reference stations are "known" by the receiver, it is thus possible to calculate the exact position of the object.
Examples of such navigation systems of the above-described type are known under the names GPS and LORAN-C. These systems are used, for instance, by ships at sea, and by aeroplanes in their approaching an airport. For a more detailed description of such systems, reference is made to the following publications:
J. J. Spilker: "Digital Communications by Satellite", Prentice Hall, New Jersey, 1977; PA1 N. Ackroyd and R. Lorimer: "Global Navigation: A GPS User's Guide", Lloyd's of London Press, London, 1990; PA1 B. Forsell: "Radio Navigation Systems", Prentice Hall, 1991, ISBN 0-13-751058-6.
The contents of these publications is incorporated in the present application by reference.
It will be evident that in the above-mentioned applications, accuracy is of utmost importance, and any error due to multipath should be eliminated or at least minimized.